I'm surprised by the reactions people are having to this... is it really worth having 24x7 access to a "perfect" signal worth ~$1,000 a year to you? I have Verizon on one iPhone and AT&T on another iPhone and I don't notice much of a difference between the two. Now and then I find a place where the AT&T phone has service and the Verizon doesn't ... other times, Verizon works and AT&T doesn't. Most of the time they both work fine.
I had Sprint for about 8 years before switching to iPhone and it worked fine for me - good, even. I could drive 8 hours/500 miles from Lubbock, TX to OKC, OK and there was only about a 2 mile stretch where it would drop a call. Literally the rest of the way I could be on the phone without any problems.
If I worked or lived or often traveled somewhere that had no Sprint service, I would not go with Sprint. Same with any other carrier. Verizon (and others) do not have some magical ability to deliver a flawless signal everywhere.
That being said, what is the worst that can happen? This is free other than taxes and fees. I'll be saving enough to buy a used MacBook or buy a new phone outright.
And for those who are saying how desperate Sprint must be, I think you should try the math. Most carriers (Verizon included) have been offering up to $650 toward each line that gets ported in. Those carriers often can be out the $650 per line with no guarantee people will stay with the service after their phones/ETFs get paid off. Sprint, on the other hand, can pay out this user acquisition fee over time by spreading the cost over 1 year. The terms of the agreement also means Sprint won't be doing any sort of promotion related to discounted phones or financed phones for 4 months, which likely means the new iPhones will come out in the meantime. This gives Sprint time to get new iPhones to their long term paying customers and then in 4 months, a lot of people who signed up for this free offer will be so interested the new iPhones that they'll buy a new iPhone through Sprint.
I don't know if this will work out for Sprint in the long run, but I talked to a few people I know who have Sprint around here who said it worked well for them and put in my order tonight for two SIM cards for our paid-off iPhones. I look forward to no cell phone bill for a year! And if it doesn't work out, the most I'll be out is an hour or so of time and some activation fees. Seems worth it to me.
(Not to mention the goodwill Sprint will build up with customers by offering them free service for a year. If that doesn't buy loyalty, what does?)